Blackberry thinning

I have a pretty big patch of blackberries. A patch we want to keep as it has the big juicy blackberries as my 3 three year old says. The patch is out of control its about 12 feet high and 20 feet long. It is taking over the blueberry bush. How should I trim it for the top down or take out the stems form the ground. And can If i put the stems in the ground in another area will they grow or is that a waste of time.

Blackberries can be managed like raspberries, that is you cut out all the canes that had fruit this year and prune the new canes down by half. They will branch out next summer and have even more berries. Old canes don't bear nearly as much fruit as the newest canes, and after 3 years they usually die anyway. And keeping the patch cleaned out and pruned makes the harvesting easier.
BBs spread mostly by rooting from the tips of canes that touch the ground which is why a wild and happy patch keeps getting bigger. If you want to reduce the patch size you really have to dig up the extra plants. But you can plant them elsewhere easily enough. I doubt if cuttings would root this late in the season.

If you want to plant some in a different area You could cut the cane off near the ground then dig around it with a pick a couple times and rip out the knarly root ball and bury it where you want them. All of tweetys advice was great. You could also cut a couple 5 or six foot paths through the patch so you will have access to alot more berries. You could cut the main canes off at about 6 feet then the side branches cut to 2-2.5 feet and next year you should get approximately 10 billion berries.

GOOD ADVICE SO FAR. If I had your patch(20 ft long and 12 ft high) I would go ahead and prune a good portion of it(maybe a third or even half) drastically this year and the other section(s) next year or the next two years. When I say prune drastically, I mean I would probably trim all the way to the ground. You will lose production on that section next year but by the second year you will have increased production and a manageable brier patch.